And I'm Robert Siegel. A dramatic development today in Boston: The FBI announced that it is looking for two men they suspect of placing the bombs that killed three people at the Boston Marathon and injured more than 170. The FBI released both video and photos of the men at the site of the bombings. Here's Special Agent Richard DesLauriers.

RICHARD DESLAURIERS: We know the public will play a critical role in identifying and locating these individuals. Somebody out there knows these individuals as friends, neighbors, coworkers, or family members of the suspects. Though it may be difficult, the nation is counting on those with information to come forward and provide it to us.

SIEGEL: For more on today's announcement and what it means for the investigation, I'm joined by NPR's Tom Gjelten. And, Tom, given that this is radio, we have to at least try to describe what we see in these videos and photos.

TOM GJELTEN, BYLINE: That's right, Robert. However, the - as you say, the FBI announced that it was putting those photos on the FBI website and enough people were so anxious to see them that the FBI website actually crashed. If people go on the FBI website, what they're going to see are pictures of two individuals. They actually look like fairly ordinary guys in their 20s, white men. One is wearing a black baseball cap. The other is wearing a white baseball cap turned backwards. Both of them are carrying backpacks.

Now, these photos are the result of the FBI and other law enforcement officials poring over thousands of hours of surveillance video, many photographs. In that process, they found these two men, came to focus on them. They are calling them suspect one and suspect two. And what's interesting is that you can see suspect two putting down a backpack at the site of the second bombing and then walking away. Very suspicious behavior.

SIEGEL: Suspect two is the one with the white baseball cap turned around.

GJELTEN: That's right.

SIEGEL: Now, the FBI has officially labeled these two men suspects. Previously, officials said they were looking for men they've seen in videos and photographs, but they weren't calling them suspects. What changed?

GJELTEN: Well, I think we can understand exactly what changed, Robert. First of all, these two individuals were seen at the site of the first bombing and separately at the site of the second bombing. One man was at one place. The other was at the other place. However, they were seen together walking down the street previously. So they came together and then each went to one of the two bombing sites. Second, the man at the first bombing site had a black backpack. The FBI has found remnants of what was they believed to be that actual backpack at the site, black nylon, and they think that's the backpack that contained the bomb.

The - and as I said before, the second man actually left the backpack at the site of the second bombing and left. He was seen using a telephone at that site. And all this happened, these things that they observed happened within minutes of the actual bombing. So you put that all together, you've got some pretty compelling evidence.

SIEGEL: And the FBI's asking the public to help them identify these two men. And as you've said, what we've seen in the images are two men in their 20s, appear to be two white men. One has facial hair. One is seen actually holding a phone that seems - the one with the white baseball cap, he's holding it in his left hand.

GJELTEN: He's holding it in the left hand so that - maybe he's left-handed.

SIEGEL: He holds his cellphone in his left hand. He could be right-handed and does that too. What happens now?

GJELTEN: Well, what happens now is that the authorities will try to analyze those facial images against their database of facial images, try to track them down. And also, that phone call that we mentioned, they were going - they are going to try to identify that particular call, where it was made and who it was made to.

SIEGEL: Because they know at what time that image was taken...

GJELTEN: They know precisely when.

SIEGEL: ...that suspect was on the phone. Tom Gjelten, thanks so much.